HDFS-7760. Document truncate for WebHDFS. Contributed by Konstantin Shvachko.

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -312,6 +312,8 @@ Release 2.7.0 - UNRELEASED
 
     HDFS-7058. Tests for truncate CLI. (Dasha Boudnik via shv)
 
+    HDFS-7760. Document truncate for WebHDFS. (shv)
+
   OPTIMIZATIONS
 
     HDFS-7454. Reduce memory footprint for AclEntries in NameNode.

hadoop-hdfs-project/hadoop-hdfs/src/site/apt/WebHDFS.apt.vm

@@ -137,6 +137,9 @@ WebHDFS REST API
     * {{{Concat File(s)}<<<CONCAT>>>}}
         (see  {{{../../api/org/apache/hadoop/fs/FileSystem.html}FileSystem}}.concat)
 
+    * {{{Truncate a File}<<<TRUNCATE>>>}}
+        (see  {{{../../api/org/apache/hadoop/fs/FileSystem.html}FileSystem}}.truncate)
+
   * HTTP DELETE
 
     * {{{Delete a File/Directory}<<<DELETE>>>}}
@@ -492,6 +495,31 @@ Transfer-Encoding: chunked
    {{{../../api/org/apache/hadoop/fs/FileSystem.html}FileSystem}}.delete
 
 
+** {Truncate a File}
+
+  * Submit a HTTP POST request.
+
++---------------------------------
+curl -i -X POST "http://<HOST>:<PORT>/webhdfs/v1/<PATH>?op=TRUNCATE&newlength=<LONG>"
++---------------------------------
+
+  The client receives a response with a {{{Boolean JSON Schema}<<<boolean>>> JSON object}}:
+
++---------------------------------
+HTTP/1.1 200 OK
+Content-Type: application/json
+Transfer-Encoding: chunked
+
+{"boolean": true}
++---------------------------------
+
+  []
+
+  See also:
+  {{{New Length}<<<newlength>>>}},
+  {{{../../api/org/apache/hadoop/fs/FileSystem.html}FileSystem}}.truncate
+
+
 ** {Status of a File/Directory}
 
   * Submit a HTTP GET request.
@@ -2274,6 +2302,24 @@ var tokenProperties =
   {{{Set Access or Modification Time}<<<SETTIMES>>>}}
 
 
+** {New Length}
+
+*----------------+-------------------------------------------------------------------+
+|| Name          | <<<newlength>>> |
+*----------------+-------------------------------------------------------------------+
+|| Description   | The size the file is to be truncated to. |
+*----------------+-------------------------------------------------------------------+
+|| Type          | long |
+*----------------+-------------------------------------------------------------------+
+|| Valid Values  | \>= 0 |
+*----------------+-------------------------------------------------------------------+
+|| Syntax        | Any long. |
+*----------------+-------------------------------------------------------------------+
+
+  See also:
+  {{{Truncate a File}<<<TRUNCATE>>>}}
+
+
 ** {Offset}
 
 *----------------+-------------------------------------------------------------------+